Method for efficient target detection from images robust to occlusion

ABSTRACT

The method for efficient target detection from images robust to occlusion disclosed by the present invention detects the presence and spatial location of a number of objects in images. It consists in (i) an off-line method to compile an intermediate representation of detection probability maps that are then used by (ii) an on-line method to construct a detection probability map suitable for detecting and localizing objects in a set of input images efficiently. The method explicitly handles occlusions among the objects to be detected and localized, and objects whose shape and configuration is provided externally, for example from an object tracker. The method according to the present invention can be applied to a variety of objects and applications by customizing the method&#39;s input functions, namely the object representation, the geometric object model, its image projection method, and the feature matching function.

FIELD OF THE INVENTION

The present invention refers to the field of human centered IT applications, in particular the present invention refers to the technical field of camera based monitoring of movements and activities of people.

STATE OF THE ART

Monitoring movements of people, and their activities more in general, is a basic requirement for a number of human centered IT applications, which span areas with impact on society and economy such as Security and Surveillance, Ambient Assisted Living, Sports Analysis and Interactive Entertainment. To minimize intrusiveness, camera based systems are of particular interest, also because video streams encode large amount of information upon the observed scene, potentially supporting a highly detailed analysis. However, extracting relevant content from images in unconstrained settings is still challenging and requires substantial progress beyond current state-of-the-art.

A major challenge here is the automatic detection of people or, more in general, of specific objects, in images. This task becomes even more challenging for interactive applications, where the detection must be achieved almost instantaneously in order to enable the application to react in real time. The current invention addresses this problem focalizing on real time performance. An analysis of state-of-the-art video analytic systems reveals that a major difficulty consists in the online detection of newly entered targets, and their characterization in terms of a visual signature to be used as a target model to track the object in subsequent images. Detection must be based on observable features that are largely invariant within the class of target objects of interest (e.g. physical shape of an object), while target properties that are specific to each instance are assembled in the signature (e.g. physical size, color properties). In this setting, the detection task is the typically more demanding one as it can not make use of strong priors available at design time to limit the search, such as temporal continuity for the tracking. The key to efficiency is then to build, at each time instance, a data driven search prior on the fly from low level cues that can be extracted quickly from images.

In the article “Detection and tracking of multiple, partially occluded humans by Bayesian combination of edgelet based part detectors” by B. Wu and R. Nevatia, International Journal of Computer Vision, 75(2), 2007, a people detector and tracking system is presented. Detection is based on the extraction of body parts that are combined using a joint likelihood model making the system able to deal with partial occlusions. Tracking is based on the association of the extracted body parts with the expected position of the tracked objects and, when no association are found, on the application of the meanshift tracker. Activation and termination of tracks rely on the confidences computed from the detection responses. In the article: “MCMC-based particle filtering for tracking a variable number of interacting targets” By Z. Khan, T. Balch, and F. Dellaert, IEEE Transactions on PAMI, 27(11), 2005, a MRF motion prior is introduced to cope with target interactions, a Markov chain Monte Carlo sampling to address the exponential complexity and an extension of the method to deal with cases in which the number of target changes over time. A detection method using particle filtering is proposed in the article “A particle filter for joint detection and tracking of color objects”, by J. Czyz, B. Ristic, and B. Macq.—Image Vision Comput., 25(8): 1271-1281, 2007. A two-state Markov chain is introduced, and the problem is translated into sequential Bayesian estimation. The observation density is based on selected discriminative Haar-like features. In the article: “BraMBLe: A Bayesian multiple-blob tracker” by M. Isard and J. MacCormick—ICCV, 2003, estimation is accomplished jointly, in an expanded space that includes a discrete dimension reporting the number of targets.

In the light of the above described technical problem and the analysis of state-of-the-art methods, the present application discloses a novel detection method which conveniently marries, in terms of computational load, the power of model based search and the efficiency of data driven detection.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 shows an example of state representation, geometric shape model and image projection method suitable for applying the method according to the present invention to a PDL (People Detection and Localization) task.

FIG. 2 shows the input image and contour likelihood function computed using the example shown in FIG. 1.

FIG. 3 shows an example of state set used for the PDL task: a regular grid is imposed on the floor plane and the centers of the cells are used to construct the support maps with the off-line method and the detection probability map with the on-line method.

FIG. 4 shows a schematic view of the off-line part of the method according to the present invention.

FIG. 5 shows a schematic view of the on-line part of the method according to the present invention.

SUMMARY OF THE INVENTION

Bayesian methods, for visual tracking, with the particle filter as its most prominent instance, have proven to work effectively in the presence of clutter, occlusions, and dynamic background. When applied to track a variable number of targets, however, they become inefficient due to the absence of strong priors. The present application discloses an efficient method for computing a target detection prior (called detection probability map hereafter) suitable for real time applications. Formally, the method is derived as the inverse of an occlusion robust image likelihood, and is therefore theoretically grounded and sound. It has the advantage of being fully integrated in the Bayesian tracking framework—as shown in the article “A sampling algorithm for occlusion robust multi target detection”, document which is not published yet and is enclosed in the present patent application—and reactive as it uses sparse features not explained by tracked objects. The method disclosed by the present invention detects the presence and spatial location of a number of objects in images. It consists in (i) an off-line method to compile an intermediate representation of detection probability maps that are then used by (ii) an on-line method to construct a detection probability map suitable for detecting and localizing objects in a set of input images efficiently. The method explicitly handles occlusions among the objects to be detected and localized, and objects whose shape and configuration is provided externally, for example from an object tracker. The method according to the present invention can be applied to a variety of objects and applications by customizing the method's input functions, namely the object representation, the geometric object model, its image projection method, and the feature matching function.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the present invention initially requires the user to provide geometric information about the capturing cameras (the 3D position of the optical centre and camera orientation, as well as intrinsic parameters of the sensor and the optical lenses; i.e. camera calibration parameters), and to perform the following steps:

-   -   1. Choose a suitable numerical representation for the object's         configuration to be recognized. As an example, if a rigid and         vertically symmetric object moves on a horizontal plane all the         object's configurations can be represented in two-dimensional         space, through a two dimensional vector describing the objects         coordinates on the horizontal plane. An instance of such         representation is hereafter referred to as the object's state.     -   2. Select a geometric object model that is parameterized with         the chosen numerical representation and implement an image         projection procedure that maps the object mode onto an image         according to a given state hypothesis. An example for people         detection and localization (hereafter referred to as PDL) is         shown in enclosed FIG. 1.     -   3. Implement a likelihood function between the rendered object         model and image features that peaks on those object states that         fit an un-occluded instance of the object in a real image. The         likelihood function must be a combination (typically a sum or a         product) of elementary functions that operate on single features         pixel. An example is briefly described in enclosed FIG. 1 and         shown in FIG. 2 and in section 4 of the article: “An         Appearance-based Particle Filter for Visual Tracking in Smart         Rooms.” By O. Lanz, P. Chippendale, R. Brunelli—LNCS 4625/2008,         Classification of Events, Activities and Relationships,         Evaluation and Workshop—CLEAR'07, Baltimore, Md., USA, May 8-9,         2007, pp. 57-69.

The above calculated functions are used by the off-line part of the method according to the present invention to compile the intermediate representations of detection probability maps, hereafter referred to as support maps. The on-line part of the method according to the present invention then uses them to process a set of live images (one image per camera; a typical set up uses three to four color cameras to monitor a squared room of size 8×8 m² in a PDL task).

A schematic view of the off-line method is shown in enclosed FIG. 4.

First, a finite number of suitable object states to be detected is chosen. Typically, for this purpose, a grid is superimposed on the state space (e.g. the floor positions in the PDL task), and the centers of the grid cells are chosen.

Then the following procedure is repeated for each state in the chosen set, and for each camera.

A PDL example is shown in FIG. 3

The state is first rendered using the image projection method and the camera's calibration information.

Then, each pixel activated by the above rendering is accessed in sequence and considered to be an active feature pixel with weight 1.

The contribution of that activated pixel to the likelihood function computed for the rendered state image (i.e. the value of its elementary function), divided by the number of activated pixels in the rendered state image (i.e. the contour length for the PLD example in FIG. 1), is then computed.

The set of all values computed this way for a given pixel are stored in a list that is called the pixel's support map (each entry here is coupled with its originating target state), and then used by the on-line method.

The list of entries is sorted according to the distance of the corresponding state to the camera, with the closest one in front of the sorted list. For the PLD task with a regular state space grid such support maps can be displayed as grey level images: examples are shown in FIG. 4.

The on-line part of the method according to the present invention provides the set up of detection probability maps from pixel maps on a set of input images.

A schematic view of the on-line method is shown in FIG. 5. Optionally, a list of states of external objects can be provided as input to the on-line method. Each such state is given a positive weight, and the sum of weights associated to the same target must equal 1 (i.e. the weighted states represent a probability distribution for each of the external objects). Such list may be delivered by an object tracker, such as the tracker implementing patent application EU 06116896.9 and U.S. Ser. No. 11/773,483, or be manually created to contain information about static scene objects such as furniture, wherein, for each external object, an image projection method must also be implemented. The online method uses this list to account for potential occlusions caused by external objects.

A pre-processing feature extraction step is first applied to each image (e.g. motion detection followed by edge detection and distance transform in the example of FIG. 5 and more examples are provided in the article “A sampling algorithm for occlusion robust multi target detection”, document which is not published yet and is attached to the present patent application).

Then, each feature image is processed as follows and repeated for all the active feature support maps of the activated pixels, for all cameras:

First, the list of object states provided externally is sorted according to their distance to the camera, with the closest one in front of the list. If no external input is provided, this list is considered to be empty.

Then, for each active feature pixel (i.e. pixels with non-zero feature value) the corresponding support map is accessed, and a unique list is created by merging the support map with the list of externally provided object states, and this list is again sorted according to camera distance.

Each external object is also associated a real number whose value is initialized to 0.

The entries of the joint list are then accessed sequentially, from front to back. If an entry belongs to an external object and the considered active feature pixel falls inside its rendered geometric model the real number is incremented with the value of the corresponding state weight. If, instead, the entry belongs to the support map the following is done: The real values r_(i) of all the external objects indexed by “i” are used to compute a new value π_(i)(1−r_(i)) which reflects the probability of occlusion by an external object on that pixel at the distance of the considered support map state from the camera.

Finally, this value is multiplied with the value of the support map entry and accumulated in the respective entry of the detection probability map corresponding to the considered support map state.

After accumulating all the active feature support maps for all cameras, the detection probability map shows a peak on those states that fit with an un-occluded object observed in the different images, as shown in the example illustrated in FIG. 5, and can therefore be used for target detection through peak detection. 

1. A method for efficient and occlusion robust target detection from images captured by cameras, said method comprising the steps of: a) collecting information about geometric and calibration parameters regarding the capturing cameras; b) choosing a suitable numerical representation for the object's configuration to be recognized; c) selecting a geometric object model that is parameterized with the chosen numerical representation; d) implementing an image projection procedure that maps the object model onto an image according to a given state hypothesis; e) implementing a likelihood function between the rendered object model and image features that peaks on those object states that fit an un-occluded instance of the object in a real image; f) compiling, with the above calculated likelihood functions, the intermediate representations of detection probability maps, hereafter referred to as pixel support maps; (off-line method) g) processing, with the above calculated likelihood functions, a set of live images to provide the set up of detection probability maps from pixel support maps computed in step f) on a set of input images. (on-line method)
 2. The method of claim 1, wherein said suitable numerical representation for the object's configuration to be recognized, in case of a rigid and vertically symmetric object moving on a horizontal plane, is a two dimensional vector describing the objects coordinates on the horizontal plane.
 3. The method of claim 1, wherein said likelihood function implemented at the previous step e) is a combination of elementary functions that operate on single feature pixels.
 4. The method of claim 3 wherein said combination is a sum or a product of elementary functions.
 5. The method of claim 1, wherein said step f) of compiling the intermediate representations of detection probability maps (off-line method) is performed as follows: h) a finite number of suitable object states to be detected is chosen; i) the state is first rendered using the image projection method and the camera's calibration information; j) each pixel activated by the above rendering is accessed in sequence and considered to be an active feature pixel with weight equal to 1; k) the contribution of said activated pixel to the likelihood function calculated for the rendered state image, divided by the number of activated pixels in the rendered state image, is computed; l) steps i) to k) are repeated for each state in the chosen set, m) the set of all values computed at the previous step for a given pixel are stored in a list that is called the pixel's support map wherein each entry is coupled with its originating target state; n) the list of entries compiled at the previous step is sorted according to the distance of the corresponding state to the camera, with the closest one in front of the sorted list; o) steps i) to n) are repeated for each camera.
 6. The method of claim 5, wherein said step h) is performed superimposing a grid on the state space and choosing the centers of the grid cells.
 7. The method of claim 5 characterized in that for PDL (People Detection and Localization) tasks with a regular state space grid, said support maps can be displayed as grey level images.
 8. The method of claim 1, wherein step g) is performed as follows: p) a pre-processing feature extraction step is applied to each image to be processed; q) if a list of states of external objects are provided as external inputs, each state of external objects is given a positive weight, and the sum of weights associated to the same target must equal 1 in a way that the weighted states represent a probability distribution for each of the external objects; r) if external inputs are provided, the list of object states provided externally is sorted according to their distance to the camera, with the closest one in front of the list. If no external input is provided, this list is considered to be empty; s) for each active feature pixel—like the pixels with non-zero value in the feature image obtained at previous step p)—the corresponding pixel support map is accessed, and a unique list is created by merging the support map with the list of externally provided object states, and this list is sorted according to camera distance; t) if present, each external object is associated a real number whose value is initialized to 0; u) the entries of the joint list are accessed sequentially, from front to back. If an entry belongs to an external object and the considered feature pixel falls inside its rendered geometric model the real number is incremented with the value of the corresponding state weight. If, instead, the entry belongs to the support map the following is done: The real values r_(i) of all the external objects indexed by “i” are used to compute a new value π_(i)(1−r_(i)) which reflects the probability of occlusion by an external object on that pixel at the distance of the considered support map state from the camera, this value is then multiplied with the value of the support map entry and accumulated in the entry of the detection probability map corresponding to the considered support map state. v) Steps s) to u) are repeated for all the active feature support maps of the activated pixels, for all cameras.
 9. The method of claim 8, wherein said pre-processing feature extraction of step p) comprise motion detection followed by edge detection and distance transform.
 10. The method of claim 8, wherein the list of states of step p) is delivered by an object tracker or provided by manual insertion wherein, for each external object, an image projection procedure is implemented.
 11. Apparatus for efficient and occlusion robust target detection comprising means for the implementation of the method according to claim
 1. 12. Computer program comprising computer program code means adapted to perform all the steps of claim 1, when said program is run on a computer.
 13. A computer readable medium having a program recorded thereon, said computer readable medium comprising computer program code means adapted to perform all the steps of claim 1, when said program is run on a computer. 